Scottish island hiding unknown girl (4)
I believe the answer is:
isla
'girl' is the definition.
(girl's name)
'scottish island hiding unknown' is the wordplay.
'scottish island' becomes 'islay' (island of the Hebrides).
'hiding' suggests deleting specific letters.
'unknown' becomes 'y' (algebra).
'islay' with 'y' taken out is 'ISLA'.
(Other definitions for isla that I've seen before include "Lisa (anag.), another girl's name" , "Woman" , "Girl" , "Scottish lady" , "lady from Scotland" .)